Noto Serif Bulgarian is a part of the huge Noto Fonts family for which Google writes: „When text is rendered by a computer, sometimes characters are displayed as “tofu”. They are little boxes to indicate your device doesn’t have a font to display the text.

Google has been developing a font family called Noto, which aims to support all languages with a harmonious look and feel. Noto is Google’s answer to tofu. The name noto is to convey the idea that Google’s goal is to see “no more tofu”. Noto has multiple styles and weights, and freely available to all. The comprehensive set of fonts and tools used in our development are available in our GitHub repositories. All Noto fonts are now licensed under OFL.”

Please notice that Noto Serif Bulgarian with modern Bulgarian letterform modell for Cyrillic by default was withdrawn from the Noto collection subsequently. So Noto Serif Bulgarian stay as unofficial version.